Conventionally, circuit units comprising a power distribution circuit configured by stacking a plurality of bus bar boards, and having fuses and relay switches assembled thereon have generally been known as a means for distributing electric power from a common on-vehicle power supply to each on-vehicle device. In order to achieve reduction in size of the circuit unit, it has been proposed to bond a bus bar with a control circuit member, or use semiconductor switching elements such as FETs in place of relays.
However, such a circuit unit is likely to generate heat due to a relatively large current flowing in the bus bars. More particularly, in view of large amounts of heat generated in the switching elements such as FETs or relay switches, it becomes a significant problem how such large amounts of heat are effectively radiated.
As a solution to this problem, Japanese Unexamined Patent Publication No. Hei. 5-343871 discloses in FIG. 1 and FIG. 2, a control box provided with a circuit pattern including electric parts and electronic parts and arranged on a heat radiating board. The heat radiating board has the shape of a flat plate. Mounting bores are provided at an edge of one side of the heat radiating board and a bolt is inserted into each mounting bore for fixing the heat radiating board to the vehicle body so that the control box is mounted on the vehicle body in a state that substantially the entire area of an outer surface of the heat radiating board is in contact with the vehicle body.